1. Field of the Invention
The present invention pertains to apparatus for measuring the hardness properties of materials. Specifically, the present invention pertains to improved apparatus for measuring the hardness of metallic materials in accordance with standard Brinell hardness tests.
2. Brief Description of the Prior Art
It is often desired to determine certain physical properties and characteristics of materials. This is particularly true in the production and treatment of metal goods where a particular range of properties, such as hardness, may be desired, specified or required. Hardness, as generally applied to the physical properties of materials such as metals, can be measured by determining resistance to penetration. Several scales or standards of reference for hardness of materials have been developed over the years. Two of the most common scales or tests are the Rockwell Hardness Test and the Brinell Hardness Test.
In the Brinell Hardness Test, a testing machine is provided capable of forcing a ball penetrator, of a specified hardness and diameter, against the material to be tested. Generally, in an instrument measuring depth of the impression, the material is first clamped against the testing machine with a force in excess of the loads to be applied to the ball during the test. During clamping, the penetrator ball is recessed and out of contact with the material. Clamping, by predeforming the specimen area, eliminates distortion or movement of the material during the test.
After the material is properly clamped, a preliminary or set load or force is applied to the penetrator ball. Then a full load or force is applied to the penetrator ball. The load or force applied is dependent upon the ball diameter and the Brinell scales called for in the tests. Then, the test load is reduced to the set load force and the depth of the impression made by the penetrator ball is measured by means of a direct reading device. In a standard Brinell machine, the diameter of the compression, which corresponds to a standard Brinell hardness number, is read with a microscope.
Since there is a mathematical relationship between maximum diameter of the impression made by the penetrator ball and the depth of penetration, depth measuring devices, such as dial indicators, may also be used for determining the sizes of the impressions. In recent years, LVDT's (linear voltage differential transducers) have also been utilized in measuring the depth of penetration. LVDT's provide a stator surrounding an armature which is reciprocal therein along a central axis. The axial or linear position of the armature produces a voltage which is indicative of such position. When used with a hardness tester, the stator of the LVDT is generally fixed or attached to the housing of the tester machine. One end of the LVDT armature is attached to the upper end of a shaft or other component, at the opposite end of which the penetrator ball is carried. Thus, the axial or linear position of the LVDT armature corresponds to the amount of penetration of the material being tested by the penetrator ball. If a differential voltage is obtained between the voltage produced on preliminary load application and the voltage obtained on the full load application, the depth of penetration can be determined. From the depth of penetration the diameter of the recess may be calculated and correlated with the appropriate Brinell hardness number. A Brinell hardness tester utilizing an LVDT is manufactured by the Wilson Instrument Division of American Chain & Cable Company, Inc.
There are number of problems associated with some of the Brinell hardness testing machines of the prior art. For one thing, the clamping force by which the material to be tested is clamped against the tester head may vary due to the weight of the particular part or material being tested. This may result in unaccounted for variations in test results. Due to temperature, friction, pressure surges in fluids by which loads are applied to the penetrator ball, etc., the preliminary or set and full loads applied to the penetrator ball may vary slightly, also resulting in unaccounted for test variations. Furthermore, in tester machines utilizing LVDT's, unaccounted for variations may occur due to the sensitivity of the LVDT's to improper alignment of the armature. If the penetrator shaft or other member to which the LVDT is attached is not stabilized and held in precise coaxial alignment, the voltage differentials produced by the LVDT will not be accurate.
A number of Brinell hardness testing machines have been developed over the years. Some of these machines are capable of making rapid hardness tests required in production line testing. However, for reasons such as those enumerated above, some of these testing machines are not totally acceptable and are not as precise and accurate as would be desired. Thus, attempts continue to be made to produce the desired test results, particularly in production-line testing.